Flores, 33, on Saturday night was riding in the front passenger seat as her boyfriend, Christopher Rodriguez, drove, with her three children sitting behind her, when someone heaved a large rock from a railroad-track overpass and onto Interstate 35 in Temple, Texas, police said.

"It was a loud impact, almost like an explosion, that went off in the car -- I didn't know what it was," Rodriguez told "Good Morning America," in an interview airing Monday. "When it impacted the car, my instinct was to turn away, not knowing what happened. When I looked over to my right I see Keila just laying there with no response."

? Courtesy of the Flores family
Keila Flores was killed when someone threw a rock onto her car from a highway overpass on Saturday, March 9, 2019.
Rodriguez pulled over and flagged down someone to help administer CPR, but Flores didn't respond. She was transported to a local hospital, where she was pronounced dead the next day.

"It's very frustrating. There're so many emotions, like anger. You just kind of want to take things into your own hands, knowing that's not the right thing," Rodriguez said on "GMA." "I know there are certain steps the police have to take. I know eventually they will catch the person who did this and justice will come."

He said it's been a struggle to explain the bizarre accident to Flores' three children, who were in the backseat when the rock struck the vehicle.

"It's something you don’t forget," Rodriguez said. "I just hope that it doesn't affect them in the long run, and that they will realize what a good mother they had."

? ABC News
Christopher Rodriguez said his girlfriend was killed when a rock crashed through their windshield on Saturday, March 9, 2019.

Flores' older brother, John Flores, described his sister as great mother, fun-loving, the life of the party.

"Regardless of us losing our sister, we're still a family. We plan to continue on the same path -- live life to the fullest and ensure that her children are enjoying life," John Flores said. "We'll talk to them, from time to time, about who she was as an individual and share those memories with them."

The victim's younger brother, Luis Flores, said the situation still feels "unreal."

"I wish it was a very long dream," Flores said. "I still don't believe it because that day we talked on the phone and we texted before everything happened. It's just unreal. I don't understand it."

? ABC News
A Texas family said their loved one was killed when someone threw a rock onto from a highway overpass on Saturday, March 9,2019.

The family said investigators interviewed a person of interest in the case, but the police have not released any additional details.

Officers with the Temple Police Department, which is investigating the case, said it is being probed as a homicide. It said anyone with information or surveillance footage from the area should contact the department immediately.

Rodriguez said he's confident authorities will find the person responsible.

"Her death won't be in vain. Justice will take over, and I do believe that they will do their job and they will find this person," Rodriguez said.